Method for improving low frequency energy of air gun source by sharp pulse sub-wave and application

Abstract

The application belongs to the technical field of air gun source data identification in marine seismic exploration process, and discloses a method for improving low-frequency energy of air gun source by sharp pulse sub-wave and application. The method comprises the following steps: calculating the time of reaching the main pulse peak value after the excitation of the volume air gun; then adjusting the excitation time of the volume air gun according to the obtained main pulse peak value time, so that the time of reaching the main pulse peak value of each volume air gun in the array produces a corresponding time delay, thereby constructing a sharp pulse source sub-wave with a relatively narrow main pulse waveform and increasing the low-frequency energy of the air gun source. The application controls the excitation time of air guns with different volumes to achieve the purpose of extending the low-frequency of the source and widening the frequency band of the source sub-wave, and generates a high-resolution seismic sub-wave with strong downward transmission capability to meet the relevant quality requirements of shallow sea middle-deep geological target exploration.

Description

Technical Field

[0001] This invention belongs to the field of air gun source data identification technology in marine seismic exploration, and particularly relates to the method and application of improving the low-frequency energy of air gun sources through sharp pulse wavelets. Background Technology

[0002] Airgun seismic sources generate seismic wavelets with strong low-frequency energy penetration capabilities, making the enhancement of low-frequency energy crucial for targeting mid-to-deep marine targets in offshore oil and gas exploration. As offshore oil and gas exploitation moves towards deeper layers, reservoir and exploration conditions become increasingly complex, leading to growing interest in airgun seismic sources rich in low-frequency energy. Currently, domestic and international methods primarily utilize multi-array planar and three-dimensional combinations to broaden the source wavelet frequency band and improve seismic wave propagation capabilities during seismic exploration by combining wavelet energy.

[0003] Combining multiple subarrays significantly improves the high-frequency response of seismic sources, but its effect on extending the source frequency band to lower frequencies is not significant. How to generate source wavelets with stronger low-frequency energy is a pressing problem in geophysics. This invention focuses on improving the penetration ability of seismic waves by extending the low-frequency response of the source while maintaining high frequencies.

[0004] Based on the above analysis, the problems and shortcomings of the existing technology are as follows:

[0005] (1) The existing technology for generating seismic wavelets has poor application effect on the exploration of geological targets in shallow and deep seas.

[0006] (2) Existing air gun arrays have poor performance in extending the ground frequency band to low frequencies, resulting in low accuracy of air gun source data for detecting medium and deep geological targets.

[0007] (3) Existing technologies for improving the low-frequency energy of air guns mainly include increasing the array capacity or using large-capacity air guns, but large-capacity air gun technology is still immature. Therefore, scholars often use three-dimensional arrays to suppress virtual reflections in order to reduce the attenuation of low-frequency components, but the effect is not obvious. At the same time, existing technologies are difficult to achieve both widening the frequency band and extending the low frequency. The effect of existing technologies often causes the wavelet frequency band to shift towards higher or lower frequencies. Summary of the Invention

[0008] To overcome the problems existing in related technologies, the present invention discloses a method and application for improving the low-frequency energy of an air gun source by using sharp pulse wavelets, which is used in a marine field broadband stereo observation system to explore and collect nearshore shallow water deep seismic reflection signals and for marine geological surveys and oil and gas exploration.

[0009] The technical solution is as follows: A method for improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation, characterized in that the method first calculates the time for the air gun of different capacities to reach the peak of the main pulse after excitation, and then adjusts the excitation time of air guns of different capacities according to the obtained peak time of the main pulse, so that the time for each air gun of different capacities in the array to reach the peak of the main pulse is delayed accordingly, thereby constructing a sharp pulse source wavelet with a narrower main pulse waveform and increasing the low-frequency energy of the air gun source;

[0010] Among them, the shape of the main pulse of the conventional air gun wavelet is a "right triangle" with a large apex angle, while the main pulse of the sharp pulse wavelet is similar to an isosceles triangle with a smaller apex angle than the conventional wavelet.

[0011] The peak value of the main pulse increased by 11.6%, the peak-to-peak value increased by 12.4%, the initial bubble ratio increased by 16.4%, and the effective bandwidth increased by 14.5% compared to conventional arrays. The low-frequency extension was approximately 2Hz.

[0012] Specifically, the following steps are included:

[0013] S1. Simulate gas gun wavelets of different capacities based on the van der Waals nonideal gas wavelet model and set the initial conditions of the model.

[0014] S2. Execute the simulation process according to the set initial conditions of the model;

[0015] S3. Analyze the simulated air gun wavelets and statistically analyze the time t from excitation to the peak value of the main pulse for air gun wavelets of different capacities. i ;

[0016] S4. Based on the simulation results, let the minimum time t be... i Let t0 be the time difference between Δt and t0 for different capacity air guns from excitation to reaching the peak of the main pulse. i ;

[0017] S5. Calculate the Δt i As the delayed excitation time of air guns of different capacities in the air gun array, the air gun wavelet simulation was then performed;

[0018] S6. Perform spectral analysis on the obtained air gun wavelet.

[0019] In step S1, the formula for the van der Waals nonideal gas gun wavelet model is expressed as follows:

[0020]

[0021] In the formula, a = 0.1404m 6 ·Pa·mol -2 b = 3.764 × 10 -5 m 3 ·mol-1 It is the van der Waals constant, T g For the effective thermodynamic temperature, R g m is the universal gas constant. g V is the mass of the gas. g For volume, P g Air gun pressure;

[0022] Effective thermodynamic temperature T g Depends on the high-pressure gas in the chamber:

[0023] T g =T w (1+P g / P c (2)

[0024] In the formula, P c =139MPa, T w Water temperature;

[0025] During the air gun firing process, according to the law of conservation of energy, the energy gained from heat transfer loss and mass transfer of the bubble must be balanced with the change in the bubble's internal energy:

[0026]

[0027] In the formula, T is the bubble temperature, P is the bubble pressure, and m b It is the mass of the gas inside the bubble, U=C v m b T represents the internal energy of the bubble, C m and C v These represent the isobaric specific heat capacity and the volumetric specific heat capacity, respectively. dQ / dt is the heat transfer rate through the bubble wall, dt is the unit time interval of bubble wall movement, dQ is the heat transferred from the bubble to the surrounding environment per unit time interval, dU is the decrease in the internal energy of the bubble per unit time, dV is the change in bubble volume per unit time, and dm is the change in the mass of the substance inside the bubble per unit time. The heat transfer coefficient k is determined by fitting the model with experimental data. The bubble heat loss rate is expressed as:

[0028]

[0029] In the formula, ΔT=T b -T w It is the bubble temperature T b and the surrounding water temperature T w The temperature difference between them, R is the bubble radius, and k is the heat transfer coefficient; using the van der Waals nonideal gas formula, the internal energy of a nonideal gas is a function of the gas temperature and volume as follows:

[0030]

[0031] The total differential formula is expressed as:

[0032]

[0033] Furthermore, the first law of thermodynamics transforms into:

[0034]

[0035] In the formula, Rg=Cp–Cv, where Cp is the molar heat capacity under constant pressure;

[0036] By introducing a throttling constant τ to measure the rate of change of gas quantity through the air gun port, the rate of change of gas quantity can be obtained.

[0037] For air guns in practical applications, the throttling constant of air guns with different capacities depends only on the size of the air chamber, and can be expressed as follows according to the power law:

[0038]

[0039] In the formula, τ0 is the port throttling constant independent of capacity, and ζ is the throttling power law exponent determined by comparing with measured data; according to the measurement and calculation results, the gas flow rate through the air gun port in any given time depends on the pressure difference inside and outside the air gun, and thus the gas release rate is expressed as:

[0040]

[0041] In the formula, m b It is the amount of gaseous substance released into the bubble, m g | t＝0 V is the total amount of gas in the chamber, and η is the ratio of the amount of gas in the bubble to the total amount; where V g It is the air chamber volume, m g It is the amount of gas in the chamber, P g It's the air gun pressure, P b It is bubble pressure;

[0042] The motion formula for the bubble wall is expressed as:

[0043]

[0044] In the formula, R is the bubble radius, and u and These are the velocity and acceleration of the bubble wall, respectively, and c is the velocity of the sound wave in the fluid medium. It is the enthalpy difference of the bubble wall, ρ ∞ It is the still water density at infinity, P b It is the bubble pressure, P ∞It is the hydrostatic pressure at infinity; the hydrostatic pressure of the bubble changes as the bubble rises due to buoyancy, therefore the rise of the bubble must be considered; the expression for the vertical rising velocity of the bubble during the rising process is:

[0045]

[0046] In the formula, z is the bubble depth, g is the gravitational acceleration constant, and R is the bubble radius. Therefore, the hydrostatic pressure P ∞ The expression is:

[0047]

[0048] In the formula, P atm It is standard atmospheric pressure, z g This refers to the air gun depth; at a distance of 1m from the air gun, the air gun wavelet signal can be expressed as:

[0049]

[0050] At low frequencies, the interaction between bubbles is not negligible; this interaction between bubbles can be regarded as the regulation of hydrostatic pressure; the interaction between bubbles causes pressure changes around the bubbles; relative to the seismic wavelength, the bubble is a point, and the pressure field around any bubble is the superposition of hydrostatic pressure and the time-varying pressure field generated by the bubble; the effective hydrostatic pressure at the i-th bubble is:

[0051]

[0052] In the formula, P ∞ It is hydrostatic pressure, ∑ k≠i ΔP ik It is the sum of the pressure contributions of all other air guns in the air gun array, ΔP ik It consists of the hydrostatic pressure disturbance on the i-th bubble caused by the k-th bubble, and the time delay and pressure characteristics on the i-th bubble caused by the k-th bubble:

[0053]

[0054] In the formula, r ik This represents the distance between the i-th bubble and the k-th bubble.

[0055] In step S1, the initial conditions of the model are:

[0056] Step 1.1: Set the initial value P of the air gun pressure. g | t＝0 Set to working pressure;

[0057] Step 1.2: Set the initial temperature inside the bubble to T. g =T w(1+P g / P c );

[0058] Step 1.3, Initial bubble volume V b | t＝0 =V g The initial radius is

[0059] Step 1.4: The initial velocity of the bubble wall is u = 0;

[0060] Step 1.5, Initial pressure P of the bubble b | t＝0 =P ∞ The initial temperature is water temperature T. w =18°, the initial mass of the substance inside the bubble is

[0061] Step 1.6: Set the placement position (x, y, z) of each air gun;

[0062] In step S2, the simulation is performed according to the set initial conditions of the model, specifically as follows:

[0063] Step 2.1: Input the initial conditions for the van der Waals nonideal gas gun wavelet model;

[0064] Step 2.2: Start the time loop and calculate the bubble volume at time t = k.

[0065] Step 2.3: Calculate the bubble pressure P at time t = k using formula (1). b ;

[0066] Step 2.4: Calculate the bubble heat loss rate using formula (4).

[0067] Step 2.5: Calculate the gas release rate using formula (9).

[0068] Step 2.6: Calculate the rate of change of bubble volume at time t = k.

[0069] Step 2.7: Calculate the rate of temperature change inside the bubble at time t = k using formula (7).

[0070] Step 2.8: Calculate the enthalpy difference of the bubble wall.

[0071] Step 2.9: Obtain the rate of change of bubble pressure by differentiating formula (1) with respect to time t.

[0072] Step 2.10: Differentiate the enthalpy difference with respect to time t to obtain...

[0073] Step 2.11: Calculate the rate of change of the bubble wall velocity at time t=k using formula (10). That is, the acceleration of the bubble wall;

[0074] Step 2.12, for Find the derivative with respect to time t to obtain...

[0075] Step 2.13: Since the air gun wavelet simulation is an iterative process, the bubble wall radius, bubble wall velocity, gas temperature, and mass of the gas inside the bubble can be obtained through second-order Taylor series expansion.

[0076] Step 2.14: Express the bubble pressure as a function of enthalpy, bubble wall velocity, and bubble radius: R0 is the distance from the bubble center to the far-field point;

[0077] Step 2.15: Repeat steps (2.1) to (2.14) until t > t max ;

[0078] Step 2.16: Calculate the far-field wavelet sound pressure of the air gun, including virtual reflections from the sea surface:

[0079] R s The distance between the air gun and the hydrophone is represented by the sea surface reflection coefficient, D1 is the distance between the air gun's reflection on the sea surface and the hydrophone, and D2 is the distance between the air gun's reflection on the sea surface and the hydrophone. It is the time delay of the air gun signal passing through D1 and D2.

[0080] In step S3, the simulated air gun wavelet is analyzed, and the time t from excitation to the peak value of the main pulse for air gun wavelets of different capacities is statistically analyzed. i Specifically, this includes: statistics

[0081] The time t for the main pulse peak to be reached after firing with gas guns of equal volumes of 45 cu.in, 70 cu.in, 100 cu.in, 150 cu.in, and 250 cu.in i .

[0082] In step S4, the difference Δt between the time from excitation to the peak of the main pulse and t0 is calculated for air guns of different capacities. iSpecifically, this includes calculating the time difference Δt between the time it takes for an air gun with a capacity of 70 cu.in, 100 cu.in, 150 cu.in, and 250 cu.in to reach the peak of the main pulse and the time it takes for an air gun with a capacity of 45 cu.in to reach the peak of the main pulse. i .

[0083] In step S6, the spectrum analysis of the obtained air gun wavelet specifically includes: calculating the peak value of the main pulse, the virtual reflection value, and the peak value of the bubble pulse of the air gun array wavelet; performing spectrum analysis on the wavelet through Fourier transform; using the maximum amplitude of -6dB as the standard for judging the effective bandwidth; obtaining the effective bandwidth of the wavelet main pulse; and obtaining the main frequency of the wavelet main pulse.

[0084] Another objective of this invention is to provide an air gun array for implementing the method of improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation, the air gun array comprising 33 working guns and 6 empty guns, with a total capacity of 4040 cu.in;

[0085] The single-gun capacity and number of guns are as follows:

[0086] 6 items, 45cu.in;

[0087] 4 entries of 70cu.in;

[0088] 10 x 100cu.in, including 2 blank guns;

[0089] 11 guns, each 150 cu.in, including 2 empty guns;

[0090] Eight 250cu.in guns, including two empty guns.

[0091] Another objective of this invention is to provide an air gun source device for marine geological surveys, implementing the method described above for increasing the low-frequency energy of the air gun source by constructing sharp pulse wavelets through delayed excitation.

[0092] Another objective of this invention is to provide a gas gun source device for oil and gas exploration, implementing the method described above for improving the low-frequency energy of the gas gun source by constructing a sharp pulse wavelet through delayed excitation.

[0093] Another objective of this invention is to provide an airgun seismic source device for a broadband three-dimensional marine field observation system, used for exploring and collecting seismic reflection signals in shallow and deep nearshore waters, and to implement the method described above for improving the low-frequency energy of the airgun seismic source by constructing sharp pulse wavelets through delayed excitation.

[0094] Combining all the above technical solutions, the advantages and positive effects of this invention are as follows:

[0095] First, in view of the technical problems existing in the prior art and the difficulty of solving these problems, and closely combining the technical solution to be protected by this invention with the results and data in the research and development process, this invention analyzes in detail how the technical solution of this invention solves the technical problems and the creative technical effects brought about after solving the problems. The specific description is as follows: This invention utilizes the relationship between the air gun capacity and the peak arrival time of the main pulse, and constructs a sharp pulse wavelet by delaying excitation, so as to achieve the purpose of extending the air gun array ground frequency band to low frequency while ensuring high frequency, and finally obtains an air gun source with rich high and low frequencies and strong energy downlink capability for medium and deep geological targets.

[0096] Secondly, considering the technical solution as a whole or from a product perspective, the technical effects and advantages of the technical solution protected by this invention are specifically described as follows: This invention achieves the purpose of extending the low-frequency range of the seismic source and broadening the source wavelet frequency band by controlling the excitation time of air guns of different capacities, thereby generating high-resolution seismic wavelets with strong downlink capability to meet the relevant quality requirements for exploration of shallow and medium-deep geological targets. The array provided by this invention is a planar array. Attached Figure Description

[0097] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure;

[0098] Figure 1 This is a flowchart of a method for improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation, provided in an embodiment of the present invention.

[0099] Figure 2 These are simulated single-shot wavelet diagrams of air guns with different capacities provided in this embodiment of the invention.

[0100] Figure 3 This is a statistical chart showing the time from excitation to reaching the peak of the main pulse obtained from the simulation of a single air gun of each capacity in the air gun array provided in the embodiment of the present invention.

[0101] Figure 4 This is a plan view of the air gun array used in the embodiments of the present invention;

[0102] Figure 5(a) is a simulated spike pulse air gun sub-wave diagram provided by an embodiment of the present invention;

[0103] Figure 5(b) is a simulated spectrum provided by an embodiment of the present invention;

[0104] Figure 6(a) is a comparison diagram of the waveforms of the simulated spike pulse wavelet (with crosshairs) and the conventional wavelet (solid line) provided in the embodiment of the present invention;

[0105] Figure 6(b) is a comparison of the spectra of the simulated spike pulse wavelet (with crosshairs) and the conventional wavelet (solid line) provided in the embodiment of the present invention;

[0106] Figure 7 This is a velocity field model of the strata actually explored in a certain block of the Bohai Sea, provided in an embodiment of the present invention;

[0107] Figure 8(a) is a wavefield imaging diagram made using the prior art Ricker wavelet provided in an embodiment of the present invention;

[0108] Figure 8(b) is a wave field image of the sharp pulse wavelet constructed using the present invention, provided in an embodiment of the present invention. Detailed Implementation

[0109] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0110] I. Explanation of the Implementation Example:

[0111] The method for improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation, provided in this embodiment of the invention, includes:

[0112] First, the time it takes for the air guns with capacities of 45.in, 70cu.in, 45.in, 100cu.in, 150cu.in, and 250cu.in to reach the peak of the main pulse after excitation is calculated. Then, the excitation time of the air guns with different capacities is adjusted according to the obtained time, so that the time for each air gun in the array to reach the peak of the main pulse is delayed accordingly. This constructs a sharp pulse source wavelet with a narrower main pulse waveform, thereby achieving the purpose of improving the low-frequency energy of the air gun source.

[0113] Example 2

[0114] As shown in Figure 1, the method for improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation, provided in this embodiment of the invention, specifically includes the following steps:

[0115] S101. Simulate gas gun wavelets of different capacities based on the van der Waals nonideal gas gas gun wavelet model, and set the initial conditions of the model.

[0116] S102. Execute the simulation process according to the initial conditions set in step S101;

[0117] S103. Analyze the simulated air gun wavelets and statistically analyze the time t from excitation to the peak value of the main pulse for air gun wavelets of different capacities. i ;

[0118] S104. According to the simulation results, the smaller the capacity t i The smaller the value, the less likely it is to be the minimum value t. i Let t0 be the time difference between Δt and t0 for different capacity air guns from excitation to reaching the peak of the main pulse. i ;

[0119] S105, calculate Δt in step S104. i As the delayed excitation time of air guns of different capacities in the air gun array, the air gun wavelet simulation was then performed;

[0120] S106. Perform spectral analysis on the air gun array wavelet obtained in step S105.

[0121] Furthermore, the formula for the van der Waals nonideal gas gun wavelet model in step S101 is expressed as follows:

[0122]

[0123] In the formula, a = 0.1404m 6 ·Pa·mol -2 b = 3.764 × 10 -5 m 3 ·mol -1 It is the van der Waals constant, T g For the effective thermodynamic temperature, R g m is the universal gas constant. g V is the mass of the gas. g P represents volume. g This refers to the air gun pressure.

[0124] Laws et al. believed that the effective thermodynamic temperature T g Depends on the high-pressure gas in the chamber:

[0125] T g =T w (1+P g / P c (2)

[0126] In the formula, P c =139MPa. T w This refers to the water temperature.

[0127] During the air gun firing process, high-pressure gas is ejected from the chamber, forming bubbles. Heat is transferred outwards through the bubble walls during this process, consistent with the characteristics of an open thermodynamic system. According to the law of conservation of energy, the energy gained from heat loss and mass transfer within the bubble must balance the change in internal energy. Therefore:

[0128]

[0129] In the formula, T is the bubble temperature, P is the bubble pressure, and m b It is the mass of the gas inside the bubble, U=C v m b T represents the internal energy of the bubble, C m and C v These represent the isobaric specific heat capacity and the volumetric specific heat capacity, respectively. dQ / dt is the heat transfer rate through the bubble wall, dt is the unit time interval of bubble wall movement, dQ is the heat transferred from the bubble to the surrounding environment per unit time interval, dU is the decrease in the internal energy of the bubble per unit time, dV is the change in bubble volume per unit time, and dm is the change in the mass of the substance inside the bubble per unit time. The heat transfer coefficient k is determined by fitting the model with experimental data. The bubble heat loss rate can be expressed as:

[0130]

[0131] In the formula, ΔT=T b -T w It is the bubble temperature T b and the surrounding water temperature T w The temperature difference between them, R is the bubble radius, and k is the heat transfer coefficient; using the van der Waals nonideal gas formula, the internal energy of a nonideal gas is a function of the gas temperature and volume as follows:

[0132]

[0133] The total differential formula is expressed as:

[0134]

[0135] Furthermore, the first law of thermodynamics transforms into:

[0136]

[0137] In the formula, Rg=Cp–Cv, where Cp is the molar heat capacity under constant pressure;

[0138] To derive the rate of change of the amount of gaseous substance Introducing the throttling constant τ, which determines the gas velocity through the air gun port; where V g It is the air chamber volume, m g It is the amount of gas in the chamber, Pg It's the air gun pressure, P b It is bubble pressure.

[0139] For practical applications of air guns, the rate and total amount of high-pressure gas released into the water are controlled by parameters such as port size and port opening time, thus affecting the air gun's wavelet performance. Since the port area is fixed, the throttling constant τ improves the consistency between the model and measured data. It is assumed that the throttling constant for air guns of different capacities depends only on the chamber size. According to the power law, it can be expressed as:

[0140]

[0141] In the formula, τ0 is the port throttling constant independent of capacity, and ζ is the throttling power-law exponent determined by comparison with measured data. According to measurement and calculation results, the gas bubbles escaping into the water can persist for several milliseconds. At any given time, the gas flow rate through the air gun port depends on the pressure difference inside and outside the air gun, thus the gas release rate can be expressed as:

[0142]

[0143] In the formula, m b It is the amount of gaseous substance released into the bubble, m g | t＝0 η is the total amount of gas in the chamber, and η is the ratio of the amount of gas in the bubble to the total amount.

[0144] The motion formula for the bubble wall can be expressed as:

[0145]

[0146] In the formula, R is the bubble radius, and u and These are the velocity and acceleration of the bubble wall, respectively, and c is the velocity of the sound wave in the fluid medium. It is the enthalpy difference of the bubble wall, ρ ∞ It is the still water density at infinity, P b It is the bubble pressure, P ∞ This is the hydrostatic pressure at infinity. The hydrostatic pressure of the bubble changes as it rises due to buoyancy, therefore the rise of the bubble must be considered. The expression for the vertical rising velocity of the bubble considering its rising process is:

[0147]

[0148] In the formula, z is the bubble depth, g is the gravitational acceleration constant, and R is the bubble radius. Therefore, the hydrostatic pressure P ∞ The expression is:

[0149]

[0150] In the formula, P atm It is standard atmospheric pressure, z g This refers to the air gun depth. At a distance of 1 meter from the air gun, the air gun wavelet signal can be expressed as:

[0151]

[0152] At low frequencies, the interaction between bubbles is not negligible. This interaction can be viewed as a regulation of the hydrostatic pressure of the fluid. The interaction between bubbles causes changes in the pressure around the bubbles. Relative to the seismic wavelength, a bubble can be considered a point; therefore, the pressure field around any arbitrary bubble is the superposition of the hydrostatic pressure and the time-varying pressure field generated by the bubble. Thus, the effective hydrostatic pressure at the i-th bubble is:

[0153]

[0154] In the formula, P ∞ It is hydrostatic pressure, ∑ k≠i ΔP ik It is the sum of the pressure contributions of all other air guns in the air gun array, ΔP ik It consists of the hydrostatic pressure disturbance on the i-th bubble caused by the k-th bubble, and the time delay and pressure characteristics on the i-th bubble caused by the k-th bubble:

[0155]

[0156] In the formula, r ik This represents the distance between the i-th bubble and the k-th bubble.

[0157] Furthermore, the initial conditions in step S1 are specifically as follows:

[0158] Step 1.1, Condition 1: Set the initial value P of the air gun pressure. g | t＝0 Set to working pressure;

[0159] Step 1.2, Condition 2: The initial temperature inside the bubble is set to T. g =T w (1+P g / P c );

[0160] Step 1.3, Condition 3: Initial volume V of the bubble b | t＝0 =V g The initial radius is

[0161] Step 1.4, Condition 4: The initial velocity of the bubble wall is u = 0;

[0162] Step 1.5, Condition 5: Initial pressure P of the bubble b | t＝0 =P ∞ The initial temperature is water temperature T. w =18°, the initial mass of the substance inside the bubble is

[0163] Step 1.6, Condition 6: Set the placement position (x, y, z) of each air gun.

[0164] The simulation process in step S2, based on the initial conditions set in step S1, specifically includes:

[0165] Step 2.1: Input the initial conditions for the van der Waals nonideal gas gun wavelet model;

[0166] Step 2.2: Start the time loop and calculate the bubble volume at time t = k.

[0167] Step 2.3: Calculate the bubble pressure P at time t = k using formula (1). b ;

[0168] Step 2.4: Calculate the bubble heat loss rate using formula (4).

[0169] Step 2.5: Calculate the gas release rate using formula (9).

[0170] Step 2.6: Calculate the rate of change of bubble volume at time t = k.

[0171] Step 2.7: Calculate the rate of temperature change inside the bubble at time t = k using formula (7).

[0172] Step 2.8: Calculate the enthalpy difference of the bubble wall.

[0173] Step 2.9: Obtain the rate of change of bubble pressure by differentiating formula (1) with respect to time t.

[0174] Step 2.10: Differentiate the enthalpy difference with respect to time t to obtain...

[0175] Step 2.11: Calculate the rate of change of the bubble wall velocity at time t = k using formula (10). The acceleration of the bubble wall;

[0176] Step 2.12, for Find the derivative with respect to time t to obtain...

[0177] Step 2.13: Since the air gun wavelet simulation is an iterative process, the bubble wall radius, bubble wall velocity, gas temperature, and mass of the gas inside the bubble can be obtained through second-order Taylor series expansion.

[0178] Step 2.14: Express the bubble pressure as a function of enthalpy, bubble wall velocity, and bubble radius: R0 is the distance from the bubble center to the far-field point;

[0179] Step 2.15: Repeat steps (2.1) to (2.14) until t > t max ;

[0180] Step 2.16: Calculate the far-field wavelet sound pressure of the air gun, including virtual reflections from the sea surface: R s The distance between the air gun and the hydrophone is represented by the sea surface reflection coefficient, D1 is the distance between the air gun's reflection on the sea surface and the hydrophone, and D2 is the distance between the air gun's reflection on the sea surface and the hydrophone. It is the time delay of the air gun signal passing through D1 and D2.

[0181] Furthermore, in step S2, the simulated air gun array wavelet in step S2 is analyzed, and the time t from excitation to reaching the peak value of the main pulse is statistically analyzed for air guns of different capacities. i That is, the time t taken to reach the peak value of the main pulse after being fired by gas guns with capacities of 45 cu.in, 70 cu.in, 100 cu.in, 150 cu.in, and 250 cu.in. i .

[0182] Furthermore, in step S4, Δt is calculated based on the statistically obtained ti from step S3. i That is, calculate the time difference Δt between the time it takes for an air gun with a capacity of 70 cu.in, 100 cu.in, 150 cu.in, and 250 cu.in to reach the peak of the main pulse and the time it takes for an air gun with a capacity of 45 cu.in to reach the peak of the main pulse. i ;

[0183] Further, the air gun array in step S5 specifically comprises: a total of 39 air guns, of which 33 are operational and 6 are unloaded. The total capacity is 4040 cu.in, with individual gun capacities and numbers of guns being 45 cu.in (6 guns), 70 cu.in (4 guns), 100 cu.in (10 guns, including 2 unloaded), 150 cu.in (11 guns, including 2 unloaded), and 250 cu.in (8 guns, including 2 unloaded). The Δt calculated in step 4... iThe delayed excitation times for air guns of different capacities are as follows: 2.5 ms for 45 cu.in and 70 cu.in air guns, 2.0 ms for 100 cu.in and 150 cu.in air guns, and no delay for the 250 cu.in air gun. The array is submerged at a depth of 7 m, and the cable is submerged at a depth of 8 m.

[0184] Further, step S6 performs spectral analysis on the wavelet of the air gun array described in step S5. Specifically, this includes: calculating the peak value of the main pulse, the virtual reflection value, and the peak value of the bubble pulse of the air gun array wavelet; performing spectral analysis on the wavelet using Fourier transform; and using the maximum amplitude of -6dB as the standard for judging the effective bandwidth. Therefore, this invention uses -6dB to obtain the effective bandwidth of the main pulse of the wavelet and obtains the main frequency of the main pulse of the wavelet.

[0185] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0186] The information interaction and execution process between the above-mentioned devices / units are based on the same concept as the method embodiments of the present invention. For details on their specific functions and technical effects, please refer to the method embodiments section, and they will not be repeated here.

[0187] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the above-described division of functional units and modules is merely an example. In practical applications, the above functions can be assigned to different functional units and modules as needed, that is, the internal structure of the device can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiments can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit. Furthermore, the specific names of the functional units and modules are only for easy differentiation and are not intended to limit the scope of protection of this invention. The specific working process of the units and modules in the above system can be referred to the corresponding process in the foregoing method embodiments.

[0188] II. Application Examples:

[0189] Application Example 1

[0190] Step 1: Simulate the air gun wavelet based on the van der Waals nonideal gas condition air gun wavelet model. First, set the initial conditions of the model. Specifically, these include an air gun placement depth of 7m, a cable placement depth of 8m, and air gun capacities and numbers of 45cu.in, 70cu.in, 100cu.in, 150cu.in, and 250cu.in, respectively. The sampling interval is 0.0005s, the seawater density is 1.03g / cm3, the seawater velocity is 1500m / s, the seawater temperature is 293.15 Kelvin, and the sea surface reflectance is -0.9, etc.

[0191] Step 2: Perform the simulation process based on the initial conditions set in Step 1. The specific execution process is as follows:

[0192] a) Input all the initial conditions for the air gun wavelet model.

[0193] b) Start the time loop and calculate the bubble volume at time t = k.

[0194] c) Calculate the bubble pressure P at time t = k using formula (1). b ;

[0195] d) Calculate the bubble heat loss rate using formula (4).

[0196] e) Calculate the gas release rate using formula (9).

[0197] f) Calculate the rate of change of bubble volume at time t = k.

[0198] g) Calculate the rate of temperature change inside the bubble at time t = k using formula (7).

[0199] h) Calculate the enthalpy difference of the bubble wall.

[0200] i) The rate of change of bubble pressure is obtained by differentiating formula (1) with respect to time t.

[0201] j) Taking the derivative of the enthalpy difference with respect to time t, we get...

[0202] k) Calculate the rate of change of the bubble wall velocity at time t = k using formula (10). The acceleration of the bubble wall;

[0203] l) To Find the derivative with respect to time t to obtain...

[0204] m) Since the air gun wavelet simulation is an iterative process, the bubble wall radius, bubble wall velocity, gas temperature, and mass of the gas inside the bubble can be obtained through second-order Taylor series expansion:

[0205] n) Express the bubble pressure as a function of enthalpy, bubble wall velocity, and bubble radius: R0 is the distance from the bubble center to the far-field point;

[0206] o) Repeat steps (a) to (n) until t > t max ;

[0207] p) Calculate the far-field wavelet sound pressure of the air gun, including virtual reflections from the sea surface:

[0208] q) R s The distance between the air gun and the hydrophone is represented by the sea surface reflection coefficient, D1 is the distance between the air gun's reflection on the sea surface and the hydrophone, and D2 is the distance between the air gun's reflection on the sea surface and the hydrophone. This represents the time delay of the airgun signal after passing through D1 and D2. After steps (a) to (q), airgun sub-waves of different capacities are obtained, and statistical results are obtained as follows: Figure 2 The different capacities of the air gun sub-waves are shown.

[0209] Step 3: Calculate the time t taken for the wavelet of different capacity air guns to reach the peak value of the main pulse from excitation. i ,like Figure 3 As shown.

[0210] Step 4: Calculate t based on the results obtained in Step 3. i Calculate Δt i ;

[0211] Step 5: Based on the set initial conditions, perform a simulation of the spike pulse wavelet air gun array. The specific process is as follows: The air gun array contains 39 guns, of which 33 are operational and 6 are unoperated. The total capacity is 4040 cu.in. The capacities and numbers of individual guns are 45 cu.in (6 guns), 70 cu.in (4 guns), 100 cu.in (10 guns, including 2 unoperated guns), 150 cu.in (11 guns, including 2 unoperated guns), and 250 cu.in (8 guns, including 2 unoperated guns). The Δt calculated in Step 4... i The delayed excitation times for air guns of different capacities are as follows: 2.5 ms for 45 cu.in and 70 cu.in air guns, 2.0 ms for 100 cu.in and 150 cu.in air guns, and no delay for the 250 cu.in air gun. The array is submerged at a depth of 7 m, and the cable is submerged at a depth of 8 m. Figure 4The diagram shown is a plan view of an air gun array. The array is: 718_4040_7_66-air-narrow pulse-3, with a capacity of 4040 cu.in. * represents a cluster of guns, o represents a single gun, and + represents an empty gun.

[0212] Step 6: Perform spectral analysis on the sharp pulse wavelet based on Fourier transform to obtain the simulated spectrum diagram provided in this embodiment of the invention, as shown in Figure 5(b). The frequencies above the dashed line in the spectrum diagram correspond to the effective frequencies. The dashed line -6dB is the boundary commonly used in the field of geophysics for determining effective frequencies.

[0213] To better demonstrate the technical superiority of the present invention, a comparison is made with the actual air gun array wavelet of a certain block in the Bohai Sea. Figure 6(a) is a waveform comparison diagram of the simulated spike pulse wavelet (with crosshairs) and the conventional wavelet (solid line) provided in the embodiment of the present invention; Figure 6(b) is a spectrum comparison diagram of the simulated spike pulse wavelet (with crosshairs) and the conventional wavelet (solid line) provided in the embodiment of the present invention. Figures 6(a)-6(b) In the equation, barn represents the pressure within the chamber of the bubble generated by the air gun. (The rest of the text appears to be a typo and can be left as is.) Figures 6(a)-6(b) The comparison of neutron waves shows that, after adopting the method of this invention, the bandwidth of the obtained air gun wavelet is broadened, and the low-frequency energy in the spectrum is enhanced. The waveform characteristics of the spike pulse wavelet are evident, and it can also be seen that the spike pulse wavelet has higher low-frequency energy than the conventional wavelet.

[0214] Application Example 2

[0215] This invention also provides a method for high-precision full-waveform wavefield imaging using obtained sharp pulse wavelets. The specific method for this full-waveform wavefield imaging includes velocity field data from actual exploration in a block of the Bohai Sea, the sharp pulse wavelet constructed in this invention, and a calculation method for wavefield imaging.

[0216] The velocity field data obtained from actual exploration in a certain block of the Bohai Sea are as follows: Figure 7 As shown, the model basically includes various geological structures such as cracks, faults, depressions, synclines, anticlines, and buried hills. Imaging the model can comprehensively reflect the quality of seismic wavelets.

[0217] Figure 8(a) shows the effect of the current conventional calculation method for the Rayleigh wavelet wavefield imaging; Figure 8(b) shows the effect of the spike pulse wavelet wavefield imaging (71_4040_7_66-air-narrow pulse-3). The conventional array is the actual air gun array used in a certain block of the Bohai Sea, array number 718_4040_7_66_air. The array used in this invention is an optimized version of this array, array number 718_4040_7_66-air-narrow pulse-3.

[0218] pass Figures 8(a)-8(b) It can be seen that the wave field imaging effect achieved by using the spike pulse wavelet constructed by the present invention is better, specifically with stronger low-frequency energy, deeper energy transmission depth, wider width, and clearer identification of the structure of medium-deep buried hills and inner areas.

[0219] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0220] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention and within the spirit and principles of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation, characterized in that, The method specifically includes the following steps: S1. Simulate gas gun wavelets of different capacities based on the van der Waals nonideal gas wavelet model and set the initial conditions of the model. S2. Execute the simulation process according to the set initial conditions of the model; S3. Analyze the simulated air gun wavelets and statistically analyze the time t from excitation to the peak value of the main pulse for air gun wavelets of different capacities. i ; S4. Based on the simulation results, let the minimum time t be... i Let t0 be the time difference between Δt and t0 for different capacity air guns from excitation to reaching the peak of the main pulse. i ; S5. Calculate the Δt i As the delayed excitation time of air guns of different capacities in the air gun array, the air gun wavelet simulation was then performed; S6. Perform spectral analysis on the obtained air gun wavelet; In step S1, the equations of the van der Waals nonideal gas gun wavelet model are expressed as follows: （1） In the formula, , It is the van der Waals constant. T g For effective thermodynamic temperature, R g It is a universal gas constant. m g For gas mass, V g For volume, P g This refers to the air gun pressure. Effective temperature T g Depends on the high-pressure gas in the chamber: （2） In the formula, , T w Water temperature; During the air gun firing process, according to the law of conservation of energy, the energy gained from heat transfer loss and mass transfer of the bubble must be balanced with the change in the bubble's internal energy: （3） In the formula, It is the temperature of the bubble. It is bubble pressure. It is the mass of the gas inside the bubble. U = C v m b T This represents the internal energy of the bubble. and C v These are specific heat capacity at constant pressure and specific heat capacity at constant volume. It is the heat transfer rate through the bubble wall. d t is the unit time interval of the bubble wall movement. d Q represents the heat transferred from the bubble to its surroundings per unit time interval. d U is the amount by which the internal energy of the bubble decreases per unit time. d V is the change in bubble volume per unit time. d m is the change in mass of the substance inside the bubble per unit time, and the heat transfer coefficient. The bubble heat loss rate is determined by fitting the model with experimental data and is expressed as: （4） In the formula, It is the temperature of the bubble. and the surrounding water temperature The temperature difference between them Where is the bubble radius and k is the heat transfer coefficient; using the van der Waals nonideal gas equation, the internal energy of a nonideal gas is a function of its temperature and volume as follows: （5） The total differential equation is expressed as: （6） Furthermore, the first law of thermodynamics is transformed into: （7） In the formula, Rg = Cp – Cv , Cp This represents the molar heat capacity under constant pressure. Throttling constant of the gas velocity introduced through the air gun port The rate of change of the amount of gaseous substance is obtained. ; For air guns in practical applications, the throttling constant of air guns with different capacities depends only on the size of the air chamber, and can be expressed as follows according to the power law: （8） In the formula, It is a port throttling constant that is independent of capacity. The throttling power law exponent is determined by comparing with measured data; based on the measurement and calculation results, the gas flow rate through the air gun port at any given time depends on the pressure difference inside and outside the air gun, and thus the gas release rate is expressed as: （9） In the formula, It is the amount of gaseous substance released into the bubbles. It is the total amount of gas in the chamber. It is the ratio of the amount of gas in the bubble to the total amount; where It is the air chamber capacity. It is the amount of matter in the gas inside the chamber. It's the air gun pressure. It is bubble pressure; The equation of motion for the bubble wall is expressed as: （10） In the formula, It is the bubble radius. and These are the velocity and acceleration of the bubble wall, respectively. It is the speed of sound waves in a fluid medium. It is the enthalpy difference of the bubble wall. It is the still water density at infinity. It is bubble pressure. It is the hydrostatic pressure at infinity; the hydrostatic pressure of the bubble changes as the bubble rises due to buoyancy, therefore the rise of the bubble must be considered; the expression for the vertical rising velocity of the bubble during the rising process is: （11） In the formula, Where is the bubble depth, and g is the gravitational acceleration constant. It is the bubble radius, therefore, hydrostatic pressure The expression is: （12） In the formula, It is standard atmospheric pressure. This refers to the air gun depth; at a distance of 1m from the air gun, the air gun wavelet signal can be expressed as: （13） At low frequencies, the interaction between bubbles is not negligible; this interaction between bubbles can be regarded as the regulation of hydrostatic pressure; the interaction between bubbles causes pressure changes around the bubbles; relative to the seismic wavelength, the bubble is a point, and the long-range pressure field around any bubble is the superposition of hydrostatic pressure and the time-varying pressure field generated by the bubble; the effective hydrostatic pressure at the i-th bubble is: （14） In the formula, It is hydrostatic pressure. It is the sum of the pressure contributions of all other air guns in the air gun array. It is the first The effect of the bubble on the first The hydrostatic pressure disturbance of the first bubble, and the first The effect of the bubble on the first Delay and pressure characteristics of each bubble on a distance scale: （15） In the formula, Indicates the first The bubble and the first The spacing between bubbles.

2. The method for improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation according to claim 1, characterized in that, In step S1, the initial conditions of the model are: Step 1.1: Set the initial value of the air gun pressure. Set to working pressure; Step 1.2: The initial temperature inside the bubble is set to... ; Step 1.3, Initial volume of the bubble The initial radius is ; Step 1.4, the initial velocity of the bubble wall is ; Step 1.5, Initial pressure of the bubble The initial temperature is the water temperature. The initial mass of the substance inside the bubble is ; Step 1.6: Set the placement position (x, y, z) of each air gun; In step S2, the simulation is performed according to the set initial conditions of the model, specifically as follows: Step 2.1: Input the initial conditions for the van der Waals nonideal gas gun wavelet model; Step 2.2: Start the time loop and calculate the bubble volume at time t=k. ; Step 2.3: Calculate the bubble pressure at time t=k using equation (1). ; Step 2.4: Calculate the bubble heat loss rate using equation (4). ; Step 2.5: Calculate the gas release rate using equation (9). ; Step 2.6, Calculation rate of change of bubble volume at time 1 ; Step 2.7: Calculate using equation (7) Rate of temperature change inside the bubble at any given time ; Step 2.8: Calculate the enthalpy difference of the bubble wall. ; Step 2.9: Solve equation (1) for time. The differential yields the rate of change of bubble pressure. ; Step 2.10: Calculate the enthalpy difference with respect to time. The derivative is obtained ; Step 2.11: Calculate using equation (10) Rate of change of velocity of the bubble wall at time t That is, the acceleration of the bubble wall; Step 2.12, for , , , , , Seeking information about time The derivative is obtained, , , , , , ; Step 2.13: Since the air gun wavelet simulation is an iterative process, the bubble wall radius, bubble wall velocity, gas temperature, and mass of the gas inside the bubble can be obtained through second-order Taylor series expansion. , , and ; Step 2.14: Express the bubble pressure as a function of enthalpy, bubble wall velocity, and bubble radius: , This is the distance from the bubble center to the far-field point; Step 2.15: Repeat steps (2.1) to (2.14) until... ; Step 2.16: Calculate the far-field wavelet sound pressure of the air gun, including virtual reflections from the sea surface: , Indicates the sea surface reflectance. The distance between the air gun and the hydrophone is... It is the distance between the image of the air gun on the sea surface and the hydrophone. It is the air gun signal passing through and The time delay.

3. The method for improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation according to claim 1, characterized in that, In step S3, the simulated air gun wavelet is analyzed, and the time t from excitation to the peak value of the main pulse for air gun wavelets of different capacities is statistically analyzed. i Specifically, this includes: statistically analyzing the time t it takes for the main pulse to reach its peak value after being fired from a gas gun with capacities of 45 cu.in, 70 cu.in, 100 cu.in, 150 cu.in, and 250 cu.in. i .

4. The method for improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation according to claim 1, characterized in that, In step S4, the difference Δt between the time from excitation to the peak of the main pulse and t0 is calculated for air guns of different capacities. i Specifically, this includes calculating the time difference Δt between the time it takes for an air gun with a capacity of 70 cu.in, 100 cu.in, 150 cu.in, and 250 cu.in to reach the peak of the main pulse and the time it takes for an air gun with a capacity of 45 cu.in to reach the peak of the main pulse. i .

5. The method for improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation according to claim 1, characterized in that, In step S6, the spectrum analysis of the obtained air gun wavelet specifically includes: calculating the peak value of the main pulse, the virtual reflection value, and the peak value of the bubble pulse of the air gun array wavelet; performing spectrum analysis on the wavelet through Fourier transform; using the maximum amplitude of -6dB as the standard for judging the effective bandwidth; obtaining the effective bandwidth of the wavelet main pulse; and obtaining the main frequency of the wavelet main pulse.

6. An air gun array that implements the method of improving the low-frequency energy of an air gun source by constructing a sharp pulse wavelet through delayed excitation as described in any one of claims 1-5, characterized in that, The air gun array includes 33 working guns and 6 empty guns, with a total capacity of 4040 cu.in; The single-gun capacity and number of guns are as follows: 6 items, 45cu.in; 4 entries of 70cu.in; 10 x 100cu.in, including 2 blank guns; 11 guns, each 150 cu.in, including 2 empty guns; Eight 250cu.in guns, including two empty guns.

7. An air gun source device for marine geological surveys, comprising the method described in any one of claims 1-5 for increasing the low-frequency energy of the air gun source by constructing a sharp pulse wavelet through delayed excitation.

8. A gas gun source device for oil and gas exploration, comprising the method described in any one of claims 1-5 for improving the low-frequency energy of the gas gun source by constructing a sharp pulse wavelet through delayed excitation.

9. An air gun source device for a broadband stereoscopic observation system in the ocean field, used for exploring and collecting seismic reflection signals in shallow and deep waters of nearshore areas, comprising the method described in any one of claims 1-5 for increasing the low-frequency energy of the air gun source by constructing sharp pulse wavelets through delayed excitation.

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